Semantic image search
Probably more of a feature idea for GNOME Photos than an app (though maybe it could be an independent "Image Search" app?).
The idea is to ship some built-in ML models with the app to recongize common objects (e.g. people, trees, buildings) and allow searching for them with text. It would recognize individual people, and allow assigning them names (and therefore search the photo library by names, or create albums of specific people).
Relevant art: the iOS Photos app does lots of on-device ML to do things like this, and I think there are ready-made datasets and models for some of this stuff out there already.